There exists a plurality of situations wherein it is desirable to bind together components having specific properties. For example, there are various circumstances in which separated tissue of a patient needs to be brought together so it can heal. Such tissue may include bone, muscle, fascia or the like that has been divided to gain access for example to the thoracic cavity, the mediastinum, the abdomen or the like.
Typically, most surgical procedures involving the heart or lungs are performed through a midline sternal incision, widely referred to as median sternotomy. After an incision is made through the skin, the sternum is cut longitudinally using specialized power saws. The cut extends the entire length of the sternum, from the sternal notch at the neck to the xyphoid. This midline cut allows the two halves of the sternum in the anterior portion of the ribcage to be spread several inches apart, giving the surgeon access to the thoracic cavity. During surgery, the two halves of the sternum are typically held apart by mechanical retractors.
Once the surgeon has finished the procedure regarding the chest cavity, the sternum needs to be closed or reapproximated. For proper healing to occur, the split sternum portions are preferably engaged in face-to-face relationship and compressed together while the sternum heals. The key to the healing process of the sternum is the proper stabilization and contact of the two severed sides together.
Heretofore, there have been many techniques used to bring the separate sides of the sternum together and maintain them in contact so the healing process can occur. In a vast majority of cases, surgeons use stainless steel wire closure devices. These closure devices are composed of a thin stainless steel wire with a diameter typically of about 0.5 to 1.5 mm coupled to a curved needle. The composite device is formed by inserting one end of the stainless steel wire into a cavity in the non-sharpened end of the curved needle which is then crimped tightly to secure the wire to the needle.
The needle is used to pass the wire through the sternum or around the sternal halves, between the ribs that connect to the sternal halves. After all the wire segments have been properly positioned, clamps positioned on each wire are sequentially picked up by the surgeon and the wires are twisted around each other.
The ends are then trimmed and the twisted junctures are twisted again to create an extra-snug closure that will ensure that the sternal bones are pressed tightly against each other to minimize bleeding and ensure proper fusing of the sternal halves into an intact sternum. Normally, the wire loops are left in place permanently. Unless problems arise which require a second surgical operation to remove the wires, they remain in place for the remainder of the patient's life, even after the sternal halves have fused together again.
Despite their widespread use, the stainless steel wires suffer from numerous drawbacks that can cause problems both to the surgeon and to the patient during the operation and to the patient after closure is completed. For example, the relatively stiff and unyielding characteristic of a stainless steel wire renders it unwieldy and sometimes difficult to manage on the operative field. Furthermore, after each wire is in place, the segment that sits below the sternal halves may press down on body tissues such as a coronary artery by-pass graft or the heart itself while the other wires are being placed. Injury to these soft tissues can hence occur from these stiff wire segments during the normal course of sternal closure.
Also, during either preparation or application, the free end of a sternal wire can stab a surgeon, scrub nurse, or assistant. This substantial problem is compounded by the fact that the wire is typically cut using a wire cutter with relatively blunt blades, which generates a chiselled point that is typically quite sharp. To reduce the risks of stab wounds to the surgeons and their assistants, clamps are now typically used to secure the free ends of any wire in a patient's chest. However, such clamps are also plagued with various drawbacks including cluttering of the operating field and being tedious and time consuming to work with and around.
Furthermore, the stainless steel sternal wires can disrupt the entire image generated in a computerized axial tomography or magnetic resonance imaging scan of the chest for the remainder of the patient's life.
Still furthermore, tightening by twisting wires together with a pair of pliers is an inexact method. The surgeon has to develop a sensitive field for how much torque needs to be applied to properly tighten the wire without breaking it. Consequently, some suture wires break during installation. A wire break requires the surgeon to undo all finished sutures and start the process all over again.
Sternal wires occasionally also break after the surgery. Such breakage can be secondary to the thinning and deformation of the steel strand by the excessive force or stresses that are sometimes applied to the loop during routine closure. For fear of breaking a wire, a surgeon may tend to undertorque the suture, resulting in less than optimal closure pressure on the sternal knit line. This, in turn, can lead to dehiscence problems.
A particularly major problem associated with steel wire sutures is that post-operative stress on the closure loops may cause the thin wires to cut into and through the bone of the sternum. Indeed, since wires inherently define a relatively small contact surface, anatomical structures may experience excessive localized pressure resulting in damage. For example, bone may fracture or experience necrosis, cartilage may tear, etc.
Typically, most of the tension resulting from the twisting procedure on the wire is applied at the anterior surface of the sternum. Routine postoperative care of cardiothoracic patients requires aggressive pulmonary rehabilitation including early ambulation. The coughing, deep breathing and movement required to attain these goals imposes substantial stresses on the sternal closure. These substantial stresses may, in turn, cause the wire loops to cut the bone in an inward direction at the posterior side of the sternum. Elderly patients or patients who have thin or osteoporotic bones are particularly susceptible to this complication.
The result is further loosening of the sternal closure which can lead to painful instability of the two sternal halves with respiratory compromise and ultimately sternal dehiscence. Instability of the sternal closure can also result in internal bleeding. This, in turn, can increase the risks of infection and/or result in macerative damage to the cartilage and associated muscle tissue with a consequent increase in post-operative discomfort and in the time required for healing. Also, if a second operation for sternal rewiring is required, it is made even more difficult by the fact that the sternal halves are often sliced into pieces by the stainless steel wires.
The problem of sternal dehiscence after closure using suture loops is known, and, various solutions have been proposed. Among these are reinforcement of the sternum by implantation of longitudinally extending wires or weaving reinforcement wires around the ribs adjacent to the sternum and then applying sutures peristernally to join the sternal halves. However, these proposed solutions tend to result in increased damage to blood vessels or other soft tissue, and also may substantially increase the time required for closing the chest. Also, if infection occurs necessitating removal of the sutures, it can be very difficult to remove the reinforcing wires.
In an effort to circumvent some of the disadvantages associated with steel wires and, more particularly, to reduce the risk of having the closure structures cut into and through the bone of the sternum, substantially flat bands have been proposed. For example, U.S. Pat. No. 4,730,615 issued to Sutherland and Vasconcellos in 1988 describes a flat band made of metal and coated with plastic, which slides through a fastener device which was referred to in the patent as a “buckle”. The band contains protruding serrations which interact in a ratcheting manner with an angled tang in the buckle. This allows the band to be pulled tight while the tang slides across the raised serrations. Subsequently, if tension exerted attempts to expand or open the loop, the angled tang presses against the shoulder of a serration, thereby preventing the band from moving in the opposite direction.
A somewhat similar structure is disclosed in U.S. Pat. No. 4,813,416 issued to Pollak and Blasnik in 1989. This patent discloses a flat stainless steel band with notches rather than serrations. The notches interact with bumps in a buckle device, to hold the band securely after the band has been pulled tight.
U.S. Pat. No. 5,356,412 issued Oct. 18, 1994 to Golds and Muth discloses a strap assembly to be looped about split portions of human tissue including a flexible elongated member and a buckle member. The buckle member includes a frame member and a clamp member rotatably mounted within the frame member for movement from a non-strap securing position to a strap securing position. The clamp member rotates to the strap securing position in response to tensional forces exerted on the strap during tensioning thereof about the tissue portions.
These band-like devices provide an increased contact surface with the sternum as compared to the steel wires, and, hence, theoretically reduce the risk of cutting into and through the bone of the sternum. However, they nevertheless suffer from various limitations which limit their utility.
For example, being substantially flat and made of relatively stiff and unyielding material, they are typically unable to fittingly contact the geometry of the sternum. Also, their geometry is such that they cannot penetrate easily through the bone and, hence, can only be positioned peristernally between the ribs. Being relatively large, they typically displace the peristernal structures such as muscles.
Furthermore, because of their flat configuration, their bending moment of inertia is polarized in a predetermined direction. Consequently, they are considered unergonomical. Typically, they are even more unwieldy and difficult to manage on the operative field than steel wires.
Still furthermore, the substantially flat shape of these bands results in relatively sharp side edges. Such sharp side edges can slice into the surrounding tissues or bones like a blade when they are pulled through behind the needle. This, in turn, may cause internal haemorrhaging and associated problems. The sharp side edges, if unprotected, also have considerable potential to slide into the fingers of the operating surgeon or assistants. Furthermore, they are capable of inflicting injury to the soft tissues below the sternum during closure.
Another type of closure system attempting to circumvent problems associated with steel wires and disclosed in the prior art uses clamps. Examples of such closure systems are disclosed, for example, in U.S. Pat. No. 4,201,215 to Crossett et al and in U.S. Pat. No. 6,217,580 issued Apr. 17, 2001 to L. Scott Levin.
The sternal clamping device disclosed in the latter patent includes a pair of opposed generally J-shaped clamp members which are laterally adjustable relative to one another and can be rigidly joined via a set of machine screws. The threaded coupling of these set screws rigidly unites the clamp members one to another without lateral shifting occurring over time.
This type of system is relatively rigid and reliable. However, the components thereof are relatively large and may cause serious pain or other ailments to the patient. It is hence typically reserved to patients having an increased risk of sternal rupture or with important risk factors for infection.
In an effort to circumvent the problem of cutting into and through bone of the sternum associated with conventional steel wires, attempts have also been made to offer radially compressible sutures offering an increased contact surface area as evidenced by U.S. Pat. No. 5,423,821 issued Jun. 13, 1995 to Michael K. Pasque, the entire contents of which are incorporated expressly hereinto by reference.
According to the Pasque patent, a strand of thin flexible suture material is used which is compressible in its radial dimension but remains strong and relatively inelastic in its longitudinal dimension. The compressibility in the radial direction results either from the hollow tubular shape or the compressible nature of the materials used. The longitudinal strength may be maintained by nylon fibres or other materials for reinforcement.
The soft suture material helps cushion, distribute and minimize the stresses and damage inflicted on the sternum or ribs post-operatively. Furthermore, when not compressed, the strand has a diameter slightly larger than the diameter of the needle. Hence, after insertion, the expandable suture material provides gentle pressure against the surrounding tissue to minimize bleeding in the needle track.
A common problem to all of the hereinabove mentioned bone binding structures is that they can only be used towards fixation of the sternal halves, i.e. for immobilizing the sternal halves in close proximity to each other. However, for osteogenesis and solid union of the sternal halves to occur, compression of the sternal halves at the break boundary must be maintained during the healing process.
Fixation is a static process whereas compression is a dynamic one. Compression is dynamic because it must be maintained during dimensional redefinition occurring at the break boundary during healing. With the hereinabove mentioned prior art structures, compression across the break boundary typically decreases substantially during the healing process.
Indeed, the width of the sternum tends to decrease due to the nature of the healing process. The above-mentioned structures cannot respond to this dimensional change and, consequently, cannot maintain compression across the facing boundaries of the divided sternum during the healing process. Thus, applied pressure decreases with time.
As stated above, not only do the large initial compression forces generated with the hereinabove mentioned devices diminish in the initial phase of bone healing, but such large forces, in themselves, are detrimental relative to the concentrated forces experienced proximal to the wires. Although the hereinabove mentioned structures provide some stability, they are deficient as a means to establish a known initial force and they never reconcile the need for continuous compressive force. Furthermore, physiological activities such as coughing contribute to the degeneration not only of the sternum but also potentially of the devices themselves.
The need for providing a binding structure capable of inducing a compression at the break boundary of the divided sternum has been recognized and addressed in U.S. Pat. No. 5,766,218 issued Jun. 16, 1998 to Richard J. Arnott. The disclosed binding device includes a strap adapted to form a loop about injured tissue and a tension member attached to the strap. The tension member is adapted to maintain a predetermined stress level in the loop which compresses the edges of the tissue together to foster healing. The tension member is preferably a shape memory effect alloy, such as Nitinol, a nickel-titanium alloy. The binding device also includes a one-way locking mechanism which keeps the strap in the loop.
Also disclosed is a method of binding together injured tissue under a compressive force to promote healing. The method comprises the steps of drawing together in close proximity opposing edges of injured tissue by tightening a strap which forms loop about the injured tissue and tightening the strap so that a tension member within the strap exerts a substantially constant tension within the strap to maintain the tissue in close proximity.
The use of so-called shape memory materials such as shape memory alloys in the medical field has been disclosed in the prior art. These alloys have different phase structures, hence, different mechanical properties, at different temperatures. Information about shape memory alloys may be found, for example, on the web site www.nitinol.com, by Nitinol Devices & components, copyright 1998.
In brief, FIGS. 6A and 6B, together, schematically illustrate a typical temperature and stress hysteresis, typical elastic stresses, σy, in phase transitions, and typical stress-strain curves for a shape-memory alloy in the austenitic and martensitic phases. At low temperature, the alloy is martensitic, and is soft and plastic, having a low σy. At a high temperature, the alloy is austenitic and tough, having a high σy.
When a martensitic alloy is heated to a temperature As, the austenitic phase begins to form. Above a temperature Af, the alloy is fully austenitic. Likewise, as an austenitic alloy is cooled to a temperature Ms, the martensitic phase begins to form. Below a temperature Mf the alloy is fully martensitic.
The temperature-dependent phase structure gives rise to shape memory. At the fully austenitic phase, under proper heat treatment and working conditions, an SMA element can be given a physical shape and “pre-programmed” to memorize that shape and resume it, whenever in the austenitic phase. The “memorized” SMA element may then be cooled to a martensitic phase and plastically deformed in the martensitic phase. But when heated back to the austenitic phase it will resume its memorized shape. The transformation temperature range between the phases is noted as TTR.
The reason for the shape memory is found in the phase structure of the alloy. Most metals deform by atomic slip. Dislocations and atomic planes slide over one another and assume a new crystal position. In the new position, the crystal has no memory of its order prior to the deformation. With increased deformation, there is generally a work-hardening effect, in which the increased tangle of dislocations makes additional deformation more difficult.
This is the case even when the increased deformation is in the direction of restoring the crystal to its original shape. However, for shape memory alloys, both transitions between the austenitic and martensitic phases and deformation in the martensitic phase change lattice angles in the crystal, uniformly for the whole crystal. The original austenitic lattice structure is “remembered” and can be restored.
FIG. 6C schematically illustrates typical phase structures of a shape-memory alloy, as functions of temperature and deformation, as follows:                in the austenitic phase, the crystal has a cubic structure, and the atoms in the lattice are arranged generally at right angles to each other;        when the austenitic crystal is cooled to a martensitic phase, a twinned lattice structure is formed;        when the twinned martensitic crystal is deformed by an amount no greater than δ, the twinned structure “stretches” so that the atoms in the lattice are arranged generally at oblique angles to each other, wherein the oblique angles are determined by the amount of deformation; and        when the deformed martensitic crystal is heated, the crystal resumes its cubic structure, wherein, again, the atoms in the lattice are arranged generally at right angles to each other.        
Another property that can be imparted to SMA elements, under proper heat treatment and working conditions, is so-called superelasticity, or Stress-Induced Martensite (SIM). With this property, a fully austenitic SMA element, at a temperature above Af, will become martensitic and plastic under high stress, and deform under the stress. When the stress is removed, the SMA element will return to the austenitic phase and to its memorized shape in the austenitic phase.
Superelasticity is also referred to as rubber-band like property, because the SMA element behaves like a rubber band or a spring, deforming under stress and resuming its original shape when the stress is removed. However, this property is present only above the temperature Af, and only when it is specifically imparted to an SMA element, by proper heat treatment and working conditions.
FIG. 6D schematically illustrates a typical cyclic transformation of a superelastic alloy, at a constant temperature above the temperature Af. The transformation between the austenitic phase and a stress-induced martensitic phase is brought about by stress and is eliminated when the stress is removed.
Binding devices disclosed in the prior art using shape-memory alloys typically suffer from numerous drawbacks. For example, the structure disclosed in U.S. Pat. No. 5,766,218 is relatively complex to manufacture and, hence, potentially less reliable and more expensive. Furthermore, the use of a strap is associated with the hereinabove mentioned disadvantages inherent to its geometry.
Other medical binding devices using shape-memory alloys typically take the form of staples or clamps for bone fixation. They are easily inserted in a martensitic phase, then deformed to an open, straight-edge state, and they resume a closed, clamped state in the body, thus forming a closure on the fracture. However, again, they suffer from disadvantages inherently associated with their geometries.
Shape memory materials have also been used, inter alia, in the production of stents. As is well known, a stent is a generally tubular mesh-like device which is useful in the treatment of stenosis, strictures or aneurysms in body conduits defining lumens such as blood vessels. Shape memory material stents are designed so as to be expanded in the austenitic phase and compressed or partially expanded in the martensitic state. The shape memory alloy is typically chosen such that stent will be in the austenitic state at body temperature.
The role of the stent being to support, repair or otherwise enhance the performance of a body lumen, stents are specifically designed to provide a relatively high resistance to radial collapse. Hence, they actually teach away from the principles of the present invention as will be hereinafter disclosed.
Accordingly, against this background, there exists a need for an improved binding structure.